1. Field of the Invention
The present invention is in the field of industrial piping and more specifically relates to an apparatus and method for producing a controlled freeze seal within a pipe for the purpose of interrupting the flow of fluid through the pipe to permit repairs or maintenance to be performed downstream of the freeze seal.
2. The Prior Art
Freeze seals have historically been used in power generation plants, shipboard steam propulsion power plants, refineries, and other industrial applications. Typically, a section of pipe was enclosed in a bag which was then filled with solid carbon dioxide. If a lower temperature was required, a copper tube was wrapped around the pipe and liquid nitrogen was allowed to flow through the tube. No effort was made in these prior art techniques to control the temperature to any specific level. Eventually the pipe temperature approached the temperature of the cooling medium. It was thought that a sufficient plug was formed when a frost line was observed extending approximately one inch beyond the copper cooling tube.
In these prior art methods, no attempt was made to monitor the temperature of the pipe wall within the area covered by the wrapped copper tube, and no permanent records were made of the temperatures reached.
Recently there has been a growing appreciation of the fact that such drastic cooling to an unnecessarily low temperature results in damage to the metallurgical structure of the pipe, causing a weak spot in the pipe wall. Such weak spots have a potential for breaking catastrophically.
Confronted by this more recent appreciation of the hazards that can result from overcooling of pipes, the present inventors embarked on an effort to find a way of producing a freeze seal without overcooling the pipe.